The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor and an imaging device equipped with the imaging lens. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera. Further, the present invention relates to an imaging device equipped with the imaging lens.
In these years, in place of cellular phones, which are mainly used for making phone calls, so-called smart phones, i.e., cellular phones with functions as portable information terminals (PDA) or personal computers, have been more widely used. In general, due to the smart phones are highly-functional as compared to the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, in a case of the smartphone, although images are still viewed through printing as of its intended use, there are new additional ways of uses, such as fabrication of images to use in games, use of images for cosmetic makeup simulation, cloth fitting simulation, and the others. These new ways of uses that were not common before have been increasing each year.
Generally, an imaging lens for mounting in a cellular phone or a smartphone that is developed for an advanced user is required to have a lens configuration with high resolution so as to be applicable to an imaging element for high resolution that is available in these years. However, in a case of the imaging lens for mounting in the camera used for the above-described applications, it is more critical to be a small size with a wide angle of view, in other words, critical to have a wide angle, than having a high resolution.
Conventionally, there have been proposed various lens configurations. Among them, an imaging lens with a three-lens configuration has been used for many devices since aberrations thereof are relatively satisfactorily corrected and suitable for downsizing the imaging lens. As for the three lenses configuration, a well-known lens configuration includes a first lens having positive refractive power; a second lens having negative refractive power; and a third lens having positive or negative refractive power. According to the lens configuration, the second lens has negative refractive power. By having the configuration, accordingly, a chromatic aberration generated in the first lens is suitably corrected by the second lens, and it is simultaneously suitable for downsizing of the imaging lens.
However, in the above-described lens configuration, since the second lens has negative refractive power, in an attempt to attain both the downsizing of the imaging lens and the wide angle, it results in refractive power of each lens tends to be relatively strong, making it necessary to increase precision level for both in fabrication and assembling of the lens. For this reason, another lens configuration has been recently proposed, in which refractive powers of the three lenses are all positive.
As the imaging lens having the three-lens configuration, for example, an imaging lens described in below mentioned number of Patent Reference has been known. According to the imaging lens disclosed in Patent Reference, by having an inflection point on the second lens and the third lens, the downsizing of the imaging lens and the wide angle are attained while correcting aberrations thereof.
Patent Reference: Japanese Patent Application Publication No. 2012-14139
According to the imaging lens described in Patent Reference, it is achievable to reduce the size thereof while correcting aberrations and distortion at a certain level. However, according to the imaging lens, due to the whole lens system tends to have a long focal length, it results in an optical axis thereof has a long length. Accordingly, it makes difficult to meet the demands for the downsizing required in recent years. In addition, as described above, with diversification of camera uses, the imaging lens is required to have even a wider angle. Although the imaging lens described in Patent Reference has a wide angle compared with conventional lenses, however, in order to achieve wider angle, its limit for performance improvement is self-evident.
Here, a difficulty of attaining both the downsizing and the wide angle while satisfactorily correcting aberrations and distortion is not a problem only specific to the imaging lens mounted in the cellular phone and the smart phone, but also, it is a common problem even for an imaging lens mounted in a relatively small device such as a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, and a network camera.
In view of the problems of the conventional technology described above, the present invention objective is to provide an imaging lens capable of attaining both downsizing and a wide angle while satisfactorily correcting aberrations and distortion, and an imaging device equipped with the imaging lens.
Further objectives and advantages of the invention are shown from the following description of the invention.